Corner layout for superjunction device

ABSTRACT

A superjunction device and methods for layout design and fabrication of a superjunction device are disclosed. A layout of active cell column structures can be configured so that a charge due to first conductivity type dopants balances out charge due to second conductivity type dopants in a doped layer in an active cell region. A layout of end portions of the active cell column structures proximate termination column structures can be configured so that a charge due to the first conductivity type dopants in the end portions and a charge due to the first conductivity type dopants in the termination column structures balances out charge due to the second conductivity type dopants in a portion of the doped layer between the termination column structures and the end portions.

PRIORITY CLAIM

This application is a divisional application claiming the benefit ofpriority of commonly assigned U.S. patent application Ser. No.12/709,114, filed Feb. 19, 2010, the entire disclosure of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to metal oxide semiconductor fieldeffect transistors (MOSFETs) and more particularly to a terminationstructure for a superjunction type MOSFET device.

BACKGROUND OF THE INVENTION

Power MOSFETs have typically been developed for applications requiringpower switching and power amplification. For power switchingapplications, the commercially available devices are typically doublediffused MOSFETs (DMOSFETs). In a typical transistor, much of thebreakdown voltage BV is supported by a drift region, which is lowlydoped in order to provide a higher breakdown voltage BV. However, thelowly doped drift region also produces high on-resistance R_(ds-on). Fora typical transistor, R_(ds-on) is proportional to BV^(2.5). R_(ds-on)therefore increases dramatically with increase in breakdown voltage BVfor a conventional transistor.

Superjunctions are a well known type of semiconductor device.Superjunction transistors provide a way to achieve low on-resistance(R_(ds-on)) while maintaining a high off-state breakdown voltage (BV).Superjunction devices include alternating P-type and N-type dopedcolumns formed in the drift region. In the OFF-state of the MOSFET, thecolumns completely deplete at relatively low voltage and thus cansustain a high breakdown voltage (the columns deplete laterally, so thatthe entire p and n columns are depleted). For a superjunction, theon-resistance R_(ds-on) increases in direct proportion to the breakdownvoltage BV, which is a much less dramatic increase than in theconventional semiconductor structure. A superjunction device maytherefore have significantly lower R_(ds-on) than a conventional MOSFETdevice for the same high breakdown voltage (BV) (or conversely may havea significantly higher BV than a conventional MOSFET for a givenR_(ds-on)).

Superjunction devices are described, e.g., in “24 mΩcm² 680 V siliconsuperjunction MOSFET”, Onishi, Y.; Iwamoto, S.; Sato, T.; Nagaoka, T.;Ueno, K.; Fujihira, T., Proceedings of the 14th International Symposiumon Power Semiconductor Devices and ICs, 2002, pages: 241-244, the entirecontents of which are incorporated herein by reference. FIG. 1 is across-sectional view of part of an active cell portion of a conventionalsuperjunction device 100. In this example, the active cell portion ofthe device 100 includes a vertical FET structure (e.g., an N-channel)formed on a suitably doped (e.g., N+) substrate 102, which acts as adrain region with a drain contact 105. A suitably-doped (e.g.,N-Epitaxial (epi) or N-drift) layer 104 is located on top of thesubstrate 102. In this example, the device 100 also includes a P-bodyregion 106, an N+ source region 108, and an N+ polysilicon gate region112. The device 100 also includes a gate contact (not shown) and asource metal 114. As seen in FIG. 1A, the superjunction structures mayinclude alternating, charge balanced P-type columns 120 and N-typecolumns 122. These columns completely deplete horizontally at a lowvoltage and so are able to withstand a high breakdown voltage in thevertical direction. The N-type columns 122 may comprise of the portionsof the N-type epitaxial layer 104 that are situated adjacent to theP-type columns 120.

A termination structure for such devices is commonly made of further Pcolumns which are laid out in a pattern that extends toward the edge orstreet of the die. For convenience, the P-columns 120 that are part ofthe active cell portion of the device 100 are referred to herein asactive cell P-columns and the P-columns that are formed in thetermination region are referred to as termination P-columns.

In a superjunction device, charge needs to be balanced everywhere,including the corner and termination regions. In the central portions ofthe active region, the P columns can be arranged in uniform parallelrows, where it is simple to arrange the charge balance. However at theedges and corners, it is more difficult to achieve charge balance, whichwould lower the BV in those regions and make the device less robust. Itwould be desirable to optimize the design for the active cell cornerregions and for the termination regions of the superjunction devices tokeep the electric field distribution constant and to keep uniform BV inthe termination region. Curved termination design is used in the cornerregion to improve BV by reducing E-field. Typically, a radius corner ofabout 150-200 mm is applied. However, matching P column layouts to thecorner regions in a charge balanced manner is challenging. It is withinthis context that embodiments of the present invention arise.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a cross-sectional view illustrating a conventionalsuperjunction type MOSFET device.

FIG. 2A is a top view of a portion at a corner of a superjunction MOSFETdevice according to a first embodiment of the present invention.

FIG. 2B is a magnified top view of a sub-portion of the portion at thecorner depicted in FIG. 2A.

FIGS. 2C-2E are top views of three divided regions n1/p1, n2/p2 andn3/p3 respectively of FIG. 2B.

FIG. 3A is a top view of a portion at a corner of a superjunction MOSFETdevice according to a second embodiment of the present invention.

FIG. 3B is a magnified top view of a sub-portion of the portion at thecorner depicted in FIG. 3A.

FIG. 4 is a top view of a portion at a corner of a superjunction MOSFETdevice according to a third embodiment of the present invention.

FIGS. 5A-5C are cross-sectional views of structures for a terminationregion in a superjunction MOSFET device according to an embodiment ofthe present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Although the following detailed description contains many specificdetails for the purposes of illustration, one of ordinary skill in theart will appreciate that many variations and alterations to thefollowing details are within the scope of the invention. Accordingly,the exemplary embodiments of the invention described below are set forthwithout any loss of generality to, and without imposing limitationsupon, the claimed invention.

Introduction

A superjunction MOSFET device with a curved corner design and straightends to the P-columns will often exhibit a low breakdown voltage due tocharge imbalance in the corner regions. Prior attempts have been made tobalance the charge in the corner region by leaving small holes or Pcolumn islands that are not connected in main P column strip.Unfortunately, such solutions can cause problems with unclampedinductive switching (UIS) or might not provide enough a largeimprovement in BV. In addition, many approaches to charge balancing incorner regions require three-dimensional modeling software. Use of suchsoftware can be expensive, complicated to use and time consuming

Solution

According to an embodiment of the present invention, the end portions ofthe layout of the active cell P-columns may be adjusted to take intoaccount the curvature at the corners to optimize the charge balance.Charge balance can also be considered in the termination region design.

It is noted that it is possible to make a superjunction device in whichthe N-type and P-type doping is reversed relative to that describedabove with respect to FIG. 1. For example, N-columns could be formed ina P-type epitaxial layer to provide charge balance in superjunctiondevice active cells or for termination. To generically refer to bothpossible types column structures used in superjunction devices the termsfirst conductivity type and second conductivity type are sometimes usedto refer to the different dopant types (i.e., P-type and N-type).

In embodiments of the present invention a simple two-dimensionalapproach can be used to design the layout of the active cell columnstructures to ensure proper charge balance at the corners of thetermination column structures. One can determine a dose Q_(imp) offirst-type dopants to be implanted per unit area in active cell columnstructures of an active cell region formed in a doped layer (sometimescalled an epi layer) of the superjunction device and in terminationcolumn structures of a termination region formed in the doped layer andsurrounding the active cell region. The doped layer can be characterizedby a thickness t and a dopant density M of second conductivity typedopants of an opposite charge type to the first conductivity typedopants. A layout of the active cell column structures can be configuredso that a charge due to the first conductivity type dopants balances outcharge due to the second conductivity type dopants in the doped layer inthe active cell region. A layout of end portions of the active cellcolumn structures proximate the termination column structures can beconfigured so that a charge due to the first conductivity type dopantsin the end portions and a charge due to the first conductivity typedopants in the termination column structures balances out charge due tothe second conductivity type dopants in the adjacent doped layer in thecorner regions.

Configuring the layout of the end portions of the column structures caninclude adjusting a configuration of a layout of end portions of activecell column structures proximate a corner of the termination columnstructures to take into account a curvature of the corner.

The shape of the layout can be adjusted by dividing a portion of thedoped layer proximate the corner into one or more regions of area A andlaying out the end portions of the active cell column structures suchthat each of the one or more regions includes an area A1 of terminationcolumn and/or active cell column structure such that, for each of theone or more regions, A1/A is a constant. The constant can be equal toM·t/Q_(imp) as will be further explained below.

There are a number of different possible ways for adjusting the layoutof the end portions of the active cell column structures proximate thecorner. For example, the layout of each end portion could include ahooked portion. Alternatively, a distance between one or more of the endportions and a nearby termination column structure could be adjusted toprovide the desired charge balance in the corner regions. Furthermore,in some embodiments, the layout of the end portions can include an edgering portion connecting the end portions of two or more adjacent activecell column structures.

Examples of embodiments of the present invention are described infurther detail below.

DETAILED DESCRIPTION

FIG. 2A is a top view of a portion 200 at a corner of a superjunctionMOSFET device including both the active region 251 and a surroundingtermination region 252 configured according to an embodiment of thepresent invention. FIG. 2B is a magnified top view of a sub-portion 202of the portion 200 depicted in FIG. 2A, which, for convenience, onlyshows two adjacent active cell P-columns 204 and 206 in the activeregion 251. The active cell P-columns 204, 206 can be characterized by awidth W and a pitch H. The active cell P-columns can be part ofsuperjunction devices configured as shown in FIG. 1. End portions 210 ofthe active cell P-columns are located proximate a first terminationP-column 208. The first termination column 208 can also be referred toas an ending ring, and is at the source potential. Subsequenttermination P-columns 209 can serve as floating guard rings forspreading the voltage across the termination region. In this example,the active cell P-columns 204, 206 and termination P-columns 208 can beformed by implantation of suitable P-type dopants into an N-type layer,e.g., an N-epitaxial layer 203. The dose Q_(imp) (in dopants per unitarea) of the implanted P-type dopant in the active cell P-columns 204,206 and the termination P-columns 208 and 209 can be made uniform sothat the implantation for both the active cell columns and thetermination columns can take place during the same implantation process,using the same masks.

Although in this example, P-type dopants are implanted in an N-typelayer 203 to form active cell P-columns 204, 206 and terminationP-columns 208 and 209, those skilled in the art will recognize thatalternatively, N-type dopants could be implanted in a P-type epi layerto form active cell and termination column structures.

As noted above, the layout of the end portions 210 of the active cellcolumn structures proximate the termination column structures can beconfigured so that a charge due to the P-type dopants in the endportions 210 and the nearby termination P-column 208 balances out chargedue to N-type dopants in the surrounding portions of the N-type layer203.

In the particular example depicted in FIGS. 2A-2B, the end portions arecurved portions 210 to account for the radius of curvature of the endingring P-columns 208 in a corner region.

The shape and size of the curved end portions 210 can be calculated bycalculating an area ratio to balance the charges in the corner regions.Specifically, the charge C_(P) due to P-type dopants should equal thecharge C_(N) due to N-type dopants. The device layout may be dividedinto portions that include P-type doped regions and regions that are notP-type doped. The regions that are not P-type doped are essentiallyN-type columns. It is noted that since the P-type dopants are implantedinto an N-type doped layer 203, each portion contains N-type dopants,and are initially N-type. However in the area that are later implantedwith P-type dopants, enough P-type dopants are implanted to overcome theinitial N-type dopants to make that area P-type. For each portion thecharges due to P-type and N-type dopants should balance, i.e.:C _(P) =C _(N)  (1)

For each portion, the total charge C_(P) due to P-type dopants may bedetermined from a dose Q_(imp) of dopants implanted per unit area (toparea) to form the P-columns 204, 206, 208 and an area A1 of the portioninto which P-type dopants are implanted.C _(P) =Q _(imp) ·A1  (2)

The charge C_(N) due to N-type dopants may be determined from the dopantdensity M of N-type dopants per unit volume in the N-type layer 203, athickness t of the N-type layer (and P-type column) and a total area Aof the portion. C_(N) includes all the N type charges, including thosein the P doped area A1. The total area A includes the both the P-typearea A1 and a non-P-type-doped area A_(N) of the portion. Thus,A=A1+A_(N) andC _(N) =M·t·A  (3)

Substituting equations (2) and (3) into equation (1) yields:Q _(imp) ·A1=M·t·A  (4)

The dose Q_(imp), the dopant density M, and thickness t can bedetermined based on other considerations of the device. Assuming thesethree quantities are fixed, equation (4) can be rewritten as:

$\begin{matrix}{\frac{A\; 1}{A} = \frac{M \cdot t}{Q_{imp}}} & (5)\end{matrix}$

As long as the ratio of P-type doped areas A1 to the total area A isheld constant according to equation (5), the P-type regions and theN-type regions are charge balanced. Therefore, the problem ofconfiguring the layout of the end portions 210 proximate the corners ofthe termination P-columns 208 can be reduced to a simple 2-dimensionalproblem.

For example, as shown in FIG. 2B, the layout of the end portions 210,the nearby N-doped layer and the proximate portion of the terminationcolumns 208 can be divided into areas, which include P-doped portionshaving areas designated p1, p2, and p3 and correspondingnon-P-type-doped portions having areas designated n1, n2, and n3.Regions n1 and p1 are shown in FIG. 2C. The regions p1 and n1 can have aslight curvature to them, but are illustrated as rectangles here forsimplicity. Similarly, the hypotenuse of the ‘triangles’ of n2 and n3may also have a slight corresponding curve to them. Regions n2 and p2,are shown in FIG. 2D and regions n3 and p3 area are shown in FIG. 2E.

In the example depicted in FIGS. 2A-2E, the shapes of the regions areselected to give the end portions 210 a hooked shape near the corner ofthe termination columns 208.

Charge can be balanced in the corner region by setting the areas of theregions p1, n1, p2, n2 p3, n3 such that

$\begin{matrix}{\frac{p\; 1}{{p\; 1} + {n\; 1}} = {\frac{p\; 2}{{p\; 2} + {n\; 2}} = {\frac{p\; 3}{{p\; 3} + {n\; 3}} = \frac{M \cdot t}{Q_{imp}}}}} & (6)\end{matrix}$

There can be some flexibility in determining the shapes of the regionsp1, n1, p2, n2, p3 and n3. The sizes, shapes and location of theimplanted areas p1, p2 and p3 can be determined using the result fromequation (6).

It should be noted that most parts of the active cell P-type columnssuch as 204 and 206 are straight and run parallel to one another acrossthe die. For these straight and parallel portions, charge balance can beeasily obtained by adjusting the width W of the P-type columns and thepitch H between adjacent P-type columns. These values are chosenaccording to equation (5) such along these straight portions, half ofeach P-type column is charge balanced with the adjacent N-type regions:

$\begin{matrix}{\frac{W/2}{H/2} = {\frac{W}{H} = \frac{M \cdot t}{Q_{imp}}}} & (7)\end{matrix}$

However, in the corner regions it becomes difficult to maintain chargebalance, which is the reason for embodiments of this invention.

FIGS. 2A through 2E illustrate only one possible example amongst manyothers for a selection of the shape of p2 and p3 areas. The shapes of p3and n3 include a hook to the end portions 210. The shapes of the p1 andn1 areas can round the end portions 210 before the hook shaped portionand the shapes of p2 and n2 can be configured to remove a wedge from theend portions 210 on the opposite side of the hook shape—in order toachieve charge balance according to equation (6). The hook bends towardsthe side closest to the center of the die. For example, as shown inFIGS. 2A and 2B—in the illustrated corner of the die—the hook bends in aclockwise direction. As seen in FIG. 2E, the area p3 can be calculatedas a combination of three areas p3 _(a), p3 _(b) and p3 _(c) that aresuitable for an implantation in a lithographical process limited only bya critical dimension (CD).

With suitably configured end portions of the active cell P-columns, thebreakdown voltage (BV) at the corner regions can match the BV in thecentral portions, thus improving the robustness and reliability of thedevice.

In another embodiment, to balance the charge and to increase the BVvoltage at the corner of the active region, a P type edge ring 304 canbe formed at the end of the active cell P-columns that connects the endportions of two or more adjacent active cell P-columns. FIG. 3A is topview of a portion of a layout 300 at a corner of a superjunction MOSFETdevice. As shown in FIG. 3A, the edge ring 304 can be formed at the endof the P-columns 310 and proximate to an ending ring 306 that includestermination P-columns. The ending ring 306 is at source potential, justlike the edge ring 304 and P-column 310. Termination floating guardrings 307 can be located outside of the ending ring 306 to spread outthe electric field. A field plate (not shown) can be formed in thetermination over the P-columns of the floating guard ring 307, e.g., bypatterned deposition of metal, e.g., aluminum. The edge ring 304, endingring 306 and floating guard rings 307 can be formed as part of theimplantation step that forms the active cell P-columns 310.

FIG. 3B is a magnified top view of a sub-portion 302 of the portion 300.The charge balancing in the region between the edge ring 304 and theending ring 306 and in the region between the end portions of the activecell P-columns 310 and the edge ring 304 can be achieved using the samemethod as described above in FIGS. 2A-2B. By way of example and not byway of limitation, the BV of this corner region can be 600V.Specifically, the end portions of the active cell P-columns 310,portions of the edge ring 304 and nearby N-doped layer can be dividedinto areas that include P-doped portions and correspondingnon-P-type-doped portions. The configuration of the P-doped portions canbe adjusted to satisfy equation (5) or an equation like equation (6) sothat the charges are balanced. A similar process may be used to balancecharge in the region between the edge ring 304 and the ending ring 306.To simplify this process, the curvature radii of the edge ring 304 andending ring 306 may be selected so that there is a fixed spacing betweenthem in the corner region. As in FIGS. 2A-2E, the end regions may bebroken up into smaller areas where p-type doped area p4 (comprising p4_(a) and p4 _(b)) and non-p-type doped area n4 are charge balanced,p-type doped area p5 and non-p-type doped area n5 are charge balanced,and p-type doped area p6 and non-p-type doped area n6 are chargebalanced. Based on equation (5), the appropriate area ratios are:

$\begin{matrix}{\frac{p\; 4}{{p\; 4} + {n\; 4}} = {\frac{p\; 5}{{p\; 5} + {n\; 5}} = {\frac{p\; 6}{{p\; 6} + {n\; 6}} = \frac{M \cdot t}{Q_{imp}}}}} & (8)\end{matrix}$

Certain features can help these regions achieve the appropriate arearatio for charge balance. For example, at the intersection of the edgering 304 and the end portions of active cell P-columns 310 in the cornerregions form an acute angle on one side and an oblique angle on theother side. On the side of the intersection forming the acute angle,there is too much area with P columns. So, a notch is removed from theend portion of the active cell P-column 310 from the acute angle side.The edge ring 304 is also made less wide in the areas near an acuteangle formed than in the areas near an oblique angle. These features canbe seen in the areas n5, p5, n6 and p6.

FIG. 4 is a top view of a portion 400 at the corner of a superjunctionMOSFET device according to another embodiment of the present invention.As shown in FIG. 4, a distance d from the end of the active cellP-columns 402 and the termination columns 404 can be adjusted to obtaincharge balance in the corner region. In this example, there are ten (10)active cell P-columns 402 in the cell region for which the distance dneeds to be adjusted in order to obtain charge balance. At the sides ofthe active cell region, away from the corners, the nearby portion of thetermination columns 404 is not curved. Consequently the distance d canbe fixed here, e.g., at about half of the pitch H between two adjacent Pcolumns 402.

According to embodiments of the present invention, the layout of theactive cell column structures (including the end portions) can betransferred to a pattern of implant of first conductivity type dopants(e.g. p-type) into the doped layer, e.g. epi layer of secondconductivity type (e.g. n-type). By way of example, and not by way oflimitation, the layouts shown in any of FIG. 2A-2B, 3A-3B or 4 could beused as the basis for an implant mask, which could be manufactured byconventional means. Ions could then be implanted through the mask.Alternatively, the layouts shown in any of FIG. 2A-2B, 3A-3B or 4 couldbe used as a basis for maskless implant using a steered beam implantsystem. The remainder of the superjunction device fabrication mayproceed in an entirely conventional fashion. Consequently, embodimentsof the present invention may readily improve existing superjunctiondevice fabrication methods and facilities at relatively low expense andrapid turnaround time. Only the corners of the layouts need be modified.

There are a number of different possible termination structures that canbe used in embodiments of the present invention. By way of example, andnot by way of limitation, three possible termination structures 500, 530and 540 for the superjunction devices are shown in FIGS. 5A-5C.

As shown in FIG. 5A, the termination structure 500 includes an N+substrate 502, which acts as a drain region with a drain metal 506. AnN-Epitaxial (epi) or N-drift layer 504 is located on top of thesubstrate 502, and a field plate 510 positioned on top of a thin oxide512. The field plate may be made of a suitable electrically conductivematerial, such as N+ polysilicon. The cross-section of the field plate510 may be characterized by two-step shape, including a gate oxideportion 512 as shown in FIG. 5A. The termination structure 500 alsoincludes a metal short 518 and a field oxide 508 for electricallyisolating the metal short 518 and the field plate 510. The metal short518 connects the field plate 510 to the P+ body contact 514 and mayinclude a metal, such as Aluminum. The field oxide 508 may be formedwith phosphosilicate glass (PSG) with low thermal oxidation. As seen inFIG. 5A, the termination structures may include an inside P column 520proximate to the cell region, e.g. ending ring 208 in FIG. 2B or endingring 306 of FIG. 3B (which can be connected to the source potential,e.g., ground or zero potential), and five floating guard ring P columns522 and N-type columns that may comprise of the portions of the N-typeepitaxial layer 504 that are situated adjacent to the P-type columns520, 522. The P columns 520, 522 include a P+ contact region 514 locatedin a P-body region 516. Next to the die edge, a channel stop field plate591 is connected to the semiconductor substrate with a Schottky stylechannel stop 592. The metal portion 593 of the channel stop field platecontacts the N-drift layer 504 of the semiconductor substrate without anN+ implant or P body implant, thus forming a Schottky style channel stop592 there. This enables a functional channel stop to be formed withoutusing any additional masks (e.g., blocking a body implant or performingN+ implant).

The termination structure 530 of FIG. 5B has a similar design as thetermination structure 500, except the P columns 522 do not include a Pbody region 516. Also, the field plates 511 do not have a gate oxideportion 512 like the field plates 510 of FIG. 5A.

As shown in FIG. 5C, the termination structure 540 has a similar designas the termination structure 500, except the P columns 522 do notinclude a P+ contact region 514 and a P body region 516.

While the above is a complete description of the preferred embodiment ofthe present invention, it is possible to use various alternatives,modifications and equivalents. For example, though MOSFET superjunctiondevices are mentioned, this invention can also apply to othersuperjunction devices. Therefore, the scope of the present inventionshould be determined not with reference to the above description butshould, instead, be determined with reference to the appended claims,along with their full scope of equivalents. Any feature, whetherpreferred or not, may be combined with any other feature, whetherpreferred or not. In the claims that follow, the indefinite article “A”,or “An” refers to a quantity of one or more of the item following thearticle, except where expressly stated otherwise. The appended claimsare not to be interpreted as including means-plus-function limitations,unless such a limitation is explicitly recited in a given claim usingthe phrase “means for.”

What is claimed is:
 1. A termination structure for a semiconductordevice, comprising: a channel stop field plate on a surface of asemiconductor material proximate an edge of the semiconductor material,wherein the channel stop field plate includes a metal portion and anisolation portion, wherein the metal portion of the channel stop fieldplate contacts the semiconductor material without a heavily dopedimplant region or a body region and thereby forming a Schottky stylechannel stop, and wherein the channel stop field plate is electricallyisolated by the isolation portion from areas of the semiconductor deviceother than where the Schottky style channel stop is formed.
 2. Thetermination structure of the claim 1 wherein the metal portion of thechannel stop field plate contacts a lowly doped portion of thesemiconductor material.
 3. The termination structure of claim 1 whereinthe termination structure is formed on a die containing a semiconductordevice, wherein the semiconductor device is a MOSFET.
 4. The terminationstructure of claim 1 wherein the termination structure is formed on adie containing a semiconductor device, wherein the semiconductor deviceis a superjunction device that includes alternating charge balancedfirst doped columns of first conductivity type and second doped columnsof second conductivity type in an active cell region.